Conventionally, laminates have been well known which include: a plastic film; and a coating formed on the plastic film and containing aluminum or its oxide, alumina, as a component. The laminates are used as a packaging material having gas barrier properties for protecting articles such as foods which are susceptible to alteration by oxygen. Many of such gas barrier coatings are formed on a plastic film by a dry process such as vacuum deposition, sputtering, ion plating, or chemical vapor deposition (CVD). Aluminum-deposited films have light shielding properties as well as gas barrier properties, and are mainly used as a packaging material intended for dry foods. Alumina-deposited films, which have transparency, can allow visual recognition of package contents, and are used as a packaging material in a wide variety of applications such as retort food packaging by making use of their characteristic of permitting check for foreign matters with a metal detector and heating with a microwave oven.
A transparent gas barrier coating composed of aluminum atoms, oxygen atoms, and sulfur atoms has also been known (Patent Literature 1: JP 2003-251732 A). JP 2003-251732 A discloses a method in which a transparent coating having gas barrier properties is formed on a plastic film by reactive sputtering using aluminum as a target and a mixed gas of hydrogen sulfide and oxygen as a reaction gas.
Additionally, the present inventors have disclosed a transparent gas barrier coating composed of a reaction product of alumina particles and a phosphorus compound (Patent Literature 2: WO 2011-122036 A1). WO 2011-122036 A1 discloses a method in which a coating liquid containing alumina particles and a phosphorus compound is applied to a plastic film, followed by drying and heat treatment to form a transparent coating having gas barrier properties on the plastic film.